1. Field of the Invention
The present invention relates to methods for constructing cutoff walls having improved impermeability. In particular, the present invention relates to a method for joining liners installed in a slurry cutoff wall.
2. Description of Related Art
Cutoff walls typically are used to impede the lateral flow of ground water. Controlling the flow of ground water is essential to preventing contamination of the water supply. Cutoff walls are often used to isolate contaminated landfills and to prevent the ground water from being contaminated through contact with buried waste. Because of their low permeability, cutoff walls have also been used to seal darns, canal systems, and dikes for flood control purposes. Other applications for cutoff walls include: de-watering and protecting deep excavation projects, hydraulically isolating lagoons and holding ponds, and enclosing oil and chemical tank farms.
As illustrated in FIG. 1, slurry cutoff walls are generally constructed by excavating a narrow trench (2-4 feet slide) with a back hoe or similar device. During the excavation process, the trench is filled with slurry and maintained at a level near the top of the trench. The use of slurry, conventionally of bentonite and water, allows excavation without the need for other lateral support. The narrow trench is then back filled with excavated soil and selected impervious materials to create a cutoff wall. The selected materials are typically commercially available clays and cement. One advantage of the slurry technique is that it is relatively inexpensive to construct such a cutoff wall.
However, one problem with the cutoff walls of the prior art is their permeability. Slurry cutoff walls generally have a permeability that reaches at best 1.times.10.sup.-6 to 5.times.10.sup.-7 cm/sec. This becomes a problem when very low permeability is required such as when isolating hazardous wastes as shown in FIG. 2. When the waste site is near an aquifer or other ground water source, the permeability must be at least 1.times.10.sup.-8 cm/sec. Very low permeability cutoff walls are also needed for flood control purposes to prevent seepage through dikes and to prevent influence on the areas surrounding the cutoff wall, as well as to prevent the collapse of dikes.
One system for improving the impermeability of the area surrounding such cutoff walls is described in U.S. Pat. No. 5,246,312. In that system, a liner manufactured from impermeable material is inserted into a slurry trench as the trench is excavated. That system discloses positioning a single sheet of liner material adjacent to the cutoff wall.
To further improve the impermeability of the cutoff walls, the liners are sometimes overlapped. However, if the overlapping portions of the liner are not joined, leakage and seepage between the overlapping portions of liner are detrimental to impermeability. Therefore, there continues to be a need for a method for efficiently joining overlapping liner material to improve cutoff wall impermeability.